Electrical energy meter for 3-wire delta service

ABSTRACT

An electronic energy meter for use with a 3-wire delta service having a resistive voltage divider for sensing and scaling the line voltages is disclosed. The resistive voltage divider includes three resistive dividers, one being associated with each phase of the electrical energy being metered. Each resistive divider includes a current limiter for reducing the current into the meter to a safe level when the meter electronics are operating at an elevated level, and a resistor for scaling the line voltage to a predetermined maximum peak-to-peak value. The scaled voltage that is generated has minimal phase shift, and therefore, does not require phase shift compensation prior to processing by a processing system of the electronic meter.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.08/384,398, filed Feb. 3, 1995, now issued as U.S. Pat. No. 5,457,621 onOct. 10, 1995, which is a continuation of U.S. patent application Ser.No. 08/259,116, filed Jun. 10, 1994, now abandoned, which is acontinuation of U.S. patent application Ser. No. 07/839,967, filed Feb.21, 1992, now abandoned.

FIELD OF INVENTION

The present invention relates generally to the field of electric utilitymeters. More particularly, the present invention relates to electronicutility watthour meters or meters utilized to meter real and reactiveenergy in both the forward and reverse directions.

BACKGROUND OF THE INVENTION

Electric utility companies and power consuming industries have in thepast employed a variety of approaches to metering electrical energy.Typically, a metering system monitors power lines through isolation andscaling components to derive polyphase input representations of voltageand current. These basic inputs are then selectively treated todetermine the particular type of electrical energy being metered.Because electrical uses can vary significantly, electric utilitycompanies have requirements for meters configured to analyze severaldifferent nominal primary voltages. The most common of these voltagesare 120, 208, 240, 277 and 480 volts RMS. Presently, available metershave a different style for each of these applications, bothelectro-mechanical and electronic. This forces the electric utilitycompanies to inventory, test and maintain many different styles ofmeters. Consequently, a need exists for reducing the number of metertypes a utility need inventory by providing a meter capable of operationover a wide dynamic range.

The problem of wide amperage dynamic range was addressed in U.S. Pat.No. 3,976,941--Milkovic. It was there recognized that solid stateelectronic meters were becoming more desirable in metering applications,however, such solid state meters had a critical drawback in theiramperage dynamic range. An effort was described to improve the amperagedynamic range of solid state meters so that such meters would beoperationally equivalent to prior electro-mechanical meters. The problemwith such meters, however, was their failure to address the multiplevoltage situation. Utility companies utilizing such meters would stillbe forced to inventory, test and maintain many different styles ofmeters in order to service the various voltages provided to customers.

It has been recognized in various meter proposals that the use of amicroprocessor would make metering operations more accurate. It will beunderstood, however, that the use of a microprocessor requires theprovision of one or more supply voltages. Power supplies capable ofgenerating a direct current voltage from the line voltage have been usedfor this purpose. Since electric utility companies have requirements forvarious nominal primary voltages, it has been necessary to provide powersupplies having individualized components in order to generate themicroprocessor supply voltages from the nominal primary voltage.

Consequently, a need exists for a single meter which is capable ofmetering electrical energy associated with nominal primary voltages inthe range from 96 to 528 volts RMS. Applicants resolve the aboveproblems through the use of a switching power supply and voltagedividers. It will be recognized that switching power supplies are known.However, the use of such a power supply in an electrical energy meter isnew. Moreover, the manner of the present invention, the particular powersupply construction and its use in an electrical energy meter is novel.

It will also be noted, in order to solve the inventory problem,designing a wide voltage range meter in the past involved the use ofvoltage transformers to sense line voltage. A significant problemassociated with the use of such transformers was the change in phaseshift and the introduction of non-linearities that would occur over awide voltage range. It was not easy to remove such a widely changingphase shift or to compensate for the non-linearities.

Consequently, a need still exists for a single meter which is capable ofmetering electrical energy associated with nominal primary voltages thatalso minimizes phase shift in the voltage sensors over a wide voltagerange.

SUMMARY OF THE INVENTION

The previously described problem is resolved and other advantages areachieved in a method and apparatus for metering electrical energy over awide range of voltages with a single meter. The wide ranging meterincludes a divider network for dividing the voltage thereby generating adivided voltage. It is preferred to generate a divided voltage that issubstantially linear with minimal phase shift over the wide dynamicrange. A processing unit processes the divided voltage and a currentcomponent to determine electrical energy metering values. The processingunit requires stable supply voltages for operation. A power supply,connected to receive the voltage component and connected to theprocessing unit, generates the supply voltages from the voltagecomponent over the wide dynamic range.

It is especially preferred for the power supply to include a transformerhaving first, second and third windings, wherein the voltage componentis provided to the first winding and wherein the second winding definesthe output of the power supply. A switching member is connected to thefirst winding for permitting and preventing the flow of current throughthe first winding. The switch member is operable in response to acontrol signal. A control member generates the control signal inresponse to the output of the power supply and is connected to the thirdwinding.

In the preferred embodiment, the control member consists of anoscillator, a current sensor for sensing the flow of the current throughthe transformer primary, comparators, and various other components. Inresponse to the sensed current, the output voltage, and an inhibitsignal from a voltage clamping circuit, the control member eithercompletely disables the switching member or supplies the switchingmember with a control signal representing the ON and OFF times that theswitching member must provide in order to maintain the proper outputvoltage of the supply.

It is also preferred for the control signal to disable the switchmember. Such an embodiment is achieved by the switching member includinga first transistor, connected between the primary winding and ground,and an oscillator, connected to the base of the first transistor, forgenerating an oscillating signal for switching the transistor on andoff. In such a device the control signal causes the output of theoscillator to disable the first transistor. It is also preferred for thecontrol member to include a current sensor for sensing the currentflowing through the primary winding and for generating a sensed currentsignal. A reference current generator generates a reference currentsignal in response to the output of the power supply. A comparatorcompares the sensed current and the reference current. In such anembodiment, it is preferred for the control signal to be generated inresponse to the comparator determining that the sensed current signalexceeds the reference current signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood, and its numerousobjects and advantages will become apparent to those skilled in the artby reference to the following detailed description of the invention whentaken in conjunction with the following drawings, in which:

FIG. 1 is a block diagram of an electronic meter constructed inaccordance with the present invention;

FIG. 2 is a schematic diagram of the resistive dividers shown in FIG. 1;

FIG. 3 is a schematic diagram of the linear power supply shown in FIG.1;

FIG. 4 is a block diagram of the power supply shown in FIG. 1;

FIG. 5 is a schematic diagram of the control and switching members shownin FIG. 4;

FIG. 6 is a schematic diagram of the startup/feedback shown in FIG. 4;and

FIG. 7 is a schematic diagram of the voltage clamp shown in FIG. 4.

DETAILED DESCRIPTION

A new and novel meter for metering electrical energy is shown in FIG. 1and generally designated 10. It is noted at the outset that this meteris constructed so that the future implementation of higher levelmetering functions can be supported.

Meter 10 is shown to include three resistive voltage divider networks12A, 12B, 12C; a first processor--an ADC/DSP (analog-to-digitalconverter/digital signal processor) chip 14; a second processor--amicrocontroller 16 which in the preferred embodiment is a MitsubishiModel 50428 microcontroller; three current sensors 18A, 18B, 18C; a 12Vswitching power supply 20 that is capable of receiving inputs in therange of 96-528V; a 5V linear power supply 22; a non-volatile powersupply 24 that switches to a battery 26 when 5V supply 22 isinoperative; a 2.5V precision voltage reference 28; a liquid crystaldisplay (LCD) 30; a 32.768 kHz oscillator 32; a 6.2208 MHz oscillator 34that provides timing signals to chip 14 and whose signal is divided by1.5 to provide a 4.1472 MHz clock signal to microcontroller 16; a 2kByte EEPROM 35; a serial communications line 36; an option connector38; and an optical communications port 40 that may be used to read themeter. The inter-relationship and specific details of each of thesecomponents is set out more fully below.

It will be appreciated that electrical energy has both voltage andcurrent characteristics. In relation to meter 10 voltage signals areprovided to resistive dividers 12A-12C and current signals are inducedin a current transformer (CT) and shunted. The output of CT/shuntcombinations 18A-18C is used to determine electrical energy.

First processor 14 is connected to receive the voltage and currentsignals provided by dividers 12A-12C and shunts 18A-18C. As will beexplained in greater detail below, processor 14 converts the voltage andcurrent signals to voltage and current digital signals, determineselectrical energy from the voltage and current digital signals andgenerates an energy signal representative of the electrical energydetermination. Processor 14 will always generate a watthour delivered(Whr Del) and, watthour received (Whr Rec), depending on the type ofenergy being metered, will generate either a volt amp reactive hourdelivered (Varhr Del)/a volt amp reactive hour received (Varhr Rec)signal or volt amp hour delivered (Vahr Del)/volt amp hour received(Vahr Rec) signal. In the preferred embodiment, each transition onconductors 42-48 (each logic transition) is representative of themeasurement of a unit of energy. Second processor 16 is connected tofirst processor 14. As will be explained in greater detail below,processor 16 receives the energy signal(s) and generates an indicationsignal representative of said energy signal.

It will be noted again that meter 10 is a wide range meter capable ofmetering over a voltage range from 96-528V. The components which enhancesuch a wide range meter include the divider network 12A-12C, which aspreviously noted are connected to receive the voltage component. Thedividers generate a divided voltage, wherein the divided voltage issubstantially linear voltage with minimal phase shift over the widedynamic range, i.e. 96-528 Volts. A processing unit (processors 14 and16) are connected to receive the divided voltage and the currentcomponent. The processing unit processes the divided voltages and thecurrent components to determine electrical energy metering values. Itwill be appreciated from the following description that processors 14and 16 require stable supply voltages to be operable. A power supply,connected to receive the voltage component and connected to processors14 and 16, generate the necessary supply voltages from the Phase Avoltage component over the wide dynamic range. Power supply 20 couldalso run off of phase B and phase C voltages or a combination of theabove. However, a combination embodiment would require additionalprotection and rectifying components.

In relation to the preferred embodiment of meter 10, currents andvoltages are sensed using conventional current transformers (CT's) andresistive voltage dividers, respectively. The appropriate multiplicationis accomplished in a new integrated circuit, i.e. processor 14.Processor 14 is essentially a programmable digital signal processor(DSP) with built in multiple analog to digital (A/D) converters. Theconverters are capable of sampling multiple input channelssimultaneously at 2400 Hz each with a resolution of 21 bits and then theintegral DSP performs various calculations on the results. For a moredetailed description of Processor 14, reference is made to a copendingapplication Ser. No. 07/839,182 filed on Feb. 21, 1991 and abandoned infavor of application Ser. No. 08/259,578, which is incorporated hereinby reference and which is owned by the same assignee as the presentapplication.

Meter 10 can be operated as either a demand meter or as a time-of-use(TOU) meter. It will be recognized that TOU meters are becomingincreasingly popular due to the greater differentiation by whichelectrical energy is billed. For example, electrical energy meteredduring peak hours will be billed differently than electrical energybilled during non-peak hours. As will be explained in greater detailbelow, first processor 14 determines units of electrical energy whileprocessor 16, in the TOU mode, qualifies such energy units in relationto the time such units were determined, i.e. the season as well as thetime of day.

All indicators and test features are brought out through the face ofmeter 10, either on LCD 30 or through optical communications port 40.Power supply 20 for the electronics is a switching power supply feedinglow voltage linear supply 22. Such an approach allows a wide operatingvoltage range for meter 10.

In the preferred embodiment of the present invention, the so-calledstandard meter components and register electronics are for the firsttime all located on a single printed circuit board (not shown) definedas an electronics assembly. This electronics assembly houses powersupplies 20, 22, 24 and 28, resistive dividers 12A-12C for all threephases, the shunt resistor portion of 18A-18C, oscillator 34, processor14, processor 16, reset circuitry, EEPROM 35, oscillator 32, opticalport components 40, LCD 30, and an option board interface 38. When thisassembly is used for demand metering, the billing data is stored inEEPROM 35. This same assembly is used for TOU metering applications bymerely utilizing battery 26 and reprogramming the configuration data inEEPROM 35. The additional time-of-use billing data is stored in theinternal RAM of processor 16, which RAM is backed by battery 26.

Consider now the various components of meter 10 in greater detail.Primary current being metered may be sensed using conventional currenttransformers. The shunt resistor portion of devices 18A-18C are locatedon the electronics assembly.

The phase voltages are brought directly to the electronic assembly whereresistive dividers 12A-12C scale these inputs to processor 14. In thepreferred embodiment, the electronic components are referenced to thevector sum of each line voltage for three wire delta systems and toearth ground for all other services. Resistive division is used todivide the input voltage so that a very linear voltage with minimalphase shift over a wide dynamic range can be obtained. This incombination with a switching power supply allows the wide voltageoperating range to be implemented.

Referring briefly to FIG. 2, each resistive divider consists of two 1Meg, 1/2 watt resistors 50/52, 54/56 and 58/60, respectively. Resistors50-60 are used to drop the line voltage at an acceptable watt loss. Eachresistor pair feeds a resistor 62, 64 and 66, respectively. Resistors62-66 are metal film resistors having a minimal temperature coefficient.This combination is very inexpensive compared to other voltage sensingtechniques. Resistors 50-60 have an operating voltage rating of 300 Vrmseach. These resistors have been individually tested with the 6 kV IEEE587 impulse waveforms to assure that the resistance is stable and thatthe devices are not destroyed. Resistors 62-66 scales the input voltageto be less than 1 Volt peak to peak to processor 14. Resistors 62-66should be in the range of from about 100 ohms to about 1 K ohms toassure this maximum voltage and maintain maximum signal.

On grounded, three wire delta systems, those components of theelectronics assembly operating on logic voltage levels (including thebattery connector) can be at an elevated voltage. In such situations,the two, 1 Meg resistor combinations (50/52, 54/56, 58/60) providecurrent limiting to the logic level electronics. The worse case currentoccurs during testing of a 480 V, 3 wire delta meter with single phaseexcitation.

It will be appreciated that energy units are calculated in processor 14primarily from multiplication of voltage and current. The preferredembodiment of processor 14, referenced above as being described incopending application Ser. No. 839,182 field on Feb. 21, 1992, andabandoned in favor of application Ser. No. 259,578, includes threeanalog to digital converters. The necessity for three converters isprimarily due to the absense of voltage transformers, present in priormeters.

The M37428 microcontroller 16 is a 6502 (a traditional 8 bitmicroprocessor) derivative with an expanded instruction set for bit testand manipulation. This microcontroller includes substantialfunctionality including internal LCD drivers (128 quadraplexedsegments), 8 kbytes of ROM, 384 bytes of RAM, a full duplex hardwareUART, 5 timers, dual clock inputs (32.768 kHz and up to 8 MHz), and alow power operating mode.

During normal operation, processor 16 receives the 4.1472 MHz clock fromprocessor 14 as described above. Such a clock signal translates to a1.0368 MHz cycle time. Upon power fail, processor 16 shifts to the32.768 kHz crystal oscillator 32. This allows low power operation with acycle time of 16.384 kHz. During a power failure, processor 16 keepstrack of time by counting seconds and rippling the time forward. Onceprocessor 16 has rippled the time forward, a WIT instruction is executedwhich places the unit in a mode where only the 32.768 kHz oscillator andthe timers are operational. While in this mode a timer is setup to "wakeup" processor 16 every 32,768 cycles to count a second.

Consider now the particulars of the power supplies shown in FIG. 1. Asindicated previously, the off-line switching supply 20 is designed tooperate over a 96-528 VAC input range. It connects directly to the PhaseA voltage alternating current (AC) line and requires no line frequencytransformer. A flyback converter serves as the basis of the circuit. Aflyback converter is a type of switching power supply.

As used herein, the "AC cycle" refers to the 60 Hz or 50 Hz input topower supply 20. The "switching cycle" refers to the 50 kHz to 140 kHzfrequency at which the switching transformer of power supply 20operates. It will be noted that other switching cycle frequences can beused.

Referring now to FIG. 4, power supply 20 for use in electronic metersincludes a transformer 300 having primary and secondary windings. Theinput voltage (Phase A Voltage) is provided to the primary winding sothat current may flow therethrough. As will be appreciated from FIG. 5,the secondary winding defines the output of the power supply. Referringback to FIG. 4, a switching member 302 is connected to the primarywinding of transformer 300. Switching member 302 permits and preventsthe flow of current through the primary winding. Switch member 302 isoperable in response to a control signal, which control signal isgenerated by control circuit 304. Controller 304 generates the controlsignal in response to a limit signal generated by the start/feedbackcircuit 306 in response to the output of power supply 20. Voltage clamp308 serves to limit the voltage applied to transformer 300 and switch302. Surge protection circuit 309 is provided at the input to protectagainst surges appearing in the Phase A voltage.

Referring now to FIG. 5, transformer 300 and switch 302 are shown ingreater detail. It will be appreciated that switch 302 is a transistor.At the beginning of each switching cycle, transistor 302 "turns on",i.e. becomes conductive, and magnetizes the core of transformer 300 byapplying voltage across the primary 310. At the end of each cycle,transistor 302 turns off and allows the energy stored in the core oftransformer 300 to flow to the output of the power supply, which"output" can be generally defined by secondary 312. Simultaneously,energy flows out of the bootstrap or tertiary winding 314 to power thecontrol circuitry 304 through the start/feedback circuit 306.

Feedback circuit 306 and controller 304 control the output of powersupply 20 by varying the ON time of transistor 302. Controller 304 willbe described in greater detail in relation to FIG. 5. Transistor 302 isconnected through inverter 316 to receive the output of an oscillatorformed from inverters 318, 320 and 322. It will be recognized that suchinverters form a ring oscillator. The oscillator has a free-runfrequency of 50 KHz. The ON time of transistor 302 may vary between 200ns and 10 μs. The OFF time is always between 8 and 10 μs. Duringoperation, the bootstrap winding 314 of transformer 300 powerscontroller 304, but this power is not available until the power supplyhas started through the start/feedback circuit 306. The control circuitis a current-mode regulator.

At the beginning of a switching cycle, transistor 302 is turned ON bythe oscillator output. If left alone, transistor 302 would also beturned OFF by the oscillator output. Transistor 302 remains ON until thecurrent in primary 310 of transformer 300 ramps up to the thresholdcurrent level I_(th) represented as a voltage V_(th). As will beexplained below, V_(th) is generated by feedback circuit 306. When theprimary current of transformer 300, represented as a voltage V_(t) andsensed by resistor 326, ramps up to the threshold level V_(th),comparator 324 terminates the ON period of the oscillator by forcing theoscillator output HIGH, which output in turn is inverted by inverter316, shutting OFF transistor 302. Transistor 302 then turns OFF untilthe next switching cycle. Since the V_(th) indirectly controls the ONtime of transistor 302, controller 304 regulates the output voltage ofthe power supply by comparing the sensed current in transformer 300 tothis threshold level.

Transistor 362 and comparator 326 can disable the oscillator. Transistor362, described in greater detail in FIG. 7, disables the oscillator whenthe line voltage exceeds 400 volts. Comparator 328 disables theoscillator when the controller 304 has insufficient voltage to properlydrive transistor 302. The voltage in controller 304, V_(c), will bedescribed in relation to FIG. 5.

Consider now feedback circuit 306, shown in FIG. 6. When connected tothe Phase A Voltage, resistor 330 slowly charges capacitor 332. The highvalue of resistor 330 and the 400 volt limit by voltage clamp 308 limitthe power dissipation of resistor 330. After a few seconds, capacitor332 charges above 13 volts. Transistors 334 and 336 then providepositive feedback to each other and snap ON. Controller 304 can run fortens of milliseconds from the charge stored in capacitor 332. Normally,power supply 20 will successfully start and begin to power itself inthis period. If it fails to start, transistors 334 and 336 turn OFF whenthe charge across capacitor 332 drops below 8.5 volts and capacitor 332again charges through resistor 330. This cycle repeats until the supplystarts.

With high input voltages and without resistor 338 (FIG. 5), the currentsourced by resistor 330 can hold the control and start-up circuits in adisabled state that does not recycle. When Capacitor 332 drops below 8.5volts, resistor 338 places a load on the control circuit supply. Thisload insures that the start-up circuit recycles properly with high inputvoltages.

As indicated above, when the primary current of transformer 300 sensedby resistor 326 ramps up to the threshold level V_(th), comparator 324can terminate the ON period of the oscillator. When the voltage oncapacitor 332 is less than 13 volts, zener diode 340 provides no voltagefeedback. Under these conditions, the base-emitter voltage of transistor336 sets the current threshold I_(th) to about 650mA. This maximumcurrent limit protects transistor 302, as well as those transistors involtage clamp 308, and prevents transformer 300 from saturating.

As the voltage on capacitor 332, which is representative of the outputvoltage of the supply, approaches the proper level, zener diode 340begins to conduct and effectively reduces the current threshold, i.e.effectively reduces V_(th). Each switching cycle will then transfersless power to the output, and the supply begins to regulate its output.

When the regulating circuitry requires ON times of transistor 302 lessthan about 400 ns, the current sense circuitry does not have time toreact to the primary current of transformer 300. In that case, theregulating circuit operates as a voltage-mode pulse width modulator.Resistor 342 (FIG. 5) generates a negative step at comparator 324 at thebeginning of each switching cycle. The regulator feedback voltage atcomparator 324, which contains little current information at thebeginning of each switching cycle, translates the step into variousinput overdrives of comparator 324, thereby driving the output ofcomparator 324 to a logic HIGH level. The propagation time of thecomparator 324 decreases with increasing overdrive, i.e. as the negativestep increases, and the circuit acts as a pulse width modulator. Thenegative step will increase due to the changing level of V_(th).

Any leakage inductance between the bootstrap winding and the outputwinding causes inaccurate tracking between the voltage on capacitor 332and the output voltage of the supply. This leakage inductance can causepoor load regulation of the supply. The bootstrap and output windingsare bifilar wound; they are tightly coupled, have little leakageinductance, and provide acceptable load regulation. Since the twowindings are in direct contact, the bootstrap winding requires Tefloninsulation to meet the isolation voltage specifications. A 100% hi-pottest during manufacture insures the integrity of the insulation.

Consider now the details of voltage clamp 308, shown in FIG. 7. A 528VAC input corresponds to 750 VDC after rectification. Switchingtransistors that can directly handle these voltages are extremelyexpensive. By using the voltage clamp of the present invention,relatively inexpensive switching transistors can be utilized.

In power supply 20, the switching member 302 is shut down during partsof the AC cycle that exceed 400 volts. The switching transistor,transistor 302, in conjunction with two other transistors 344 and 346,can hold off 750 VDC. During surge conditions, these three transistorscan withstand over 1500 volts. In the preferred embodiment, transistors302, 344 and 346 are 600-volt MOSFETs.

Because high-voltage electrolytic capacitors are expensive and large,this voltage clamp 308 has no bulk filter capacitor after the bridgerectifier 348. Without a bulk filter capacitor, this switching convertermust shut down during parts of the AC cycle. It intentionally shuts downduring parts of the AC cycle that exceed 400 volts, and no input poweris available when the AC cycle crosses zero. The 2200 μF outputcapacitor 350 (FIG. 5), provides output current during these periods.

As discussed above, transistors 344 and 346 act as a voltage clamp andlimit the voltage applied to switching member 302. At a 528 VAC linevoltage, the input to the clamping circuit reaches 750 volts. Duringlightning-strike surges, this voltage may approach 1500 volts. When thevoltage at the output of bridge rectifier 348 exceeds 400 volts, zenerdiodes 352 and 354 begin to conduct. These diodes, along with the 33 KΩresistors 356, 358 and 360, create bias voltages for transistors 344 and346. Transistors 344 and 346 act as source followers and maintain theirsource voltages a few volts below their gate voltages.

If, for example, the output of bridge rectifier 348 is at 1000 volts,the gates of transistors 344 and 346 will be at approximately 400 and700 volts respectively. The source of transistor 344 applies roughly 700volts to the drain of 346; the source of 346 feeds about 400 volts toswitching member 302. Transistors 344 and 346 each drop 300 volts underthese conditions and thereby share the drop from the 1000 volt input tothe 400 volt output, a level which the switching converter 302 canwithstand.

As zener diodes 352 and 354 begin to conduct and as transistors 344 and346 begin to clamp, transistor 362 turns ON and shuts down the switchingconverter. Although transistors 344 and 346 limit the voltage fed to theconverter to an acceptable level, they would dissipate an excessiveamount of heat if the switching converter 302 consumed power during theclamping period.

When switching converter 302 shuts down, transistor 302 no longer has towithstand the flyback voltage from transformer 300. Resistor 364 takesadvantage of this by allowing the output voltage of the clamp toapproach 500 volts (instead of 400 volts) as the input to the clampapproaches 1500 volts. This removes some of the burden from transistors344 and 346.

Zener diodes 352 and 354 are off and the converter 302 runs when theoutput of bridge rectifier 348 is below 400 volts. During these parts ofthe AC cycle, the 33 KΩ resistors 356, 358 and 360 directly bias thegates of transistors 344 and 346. The voltage drop across transistors344 and 346 is then slightly more than the threshold voltages of thosetransistors along with any voltage drop generated by the channelresistance of those transistors.

During the off time of transistor 302, about 10 μS, the 33 KΩ resistorscan no longer bias the gates of transistors 344 and 346. Diode 366prevents the gate capacitance of transistors 344 and 346 and thejunction capacitance of zeners 368 and 370 from discharging whentransistor 302 is off. This keeps transistors 344 and 346 ON and readyto conduct when transistor 302 turns ON at the next switching cycle. Ifthe gates of transistors 344 and 346 had discharged between switchingcycles, they would create large voltage drops and power losses duringthe time required to recharge their gates through the 33 KΩ resistors.

In the preferred embodiment, two 33 KΩ resistors are used in series toobtain the necessary voltage capability from 966 surface-mount packages.

This power supply must withstand an 8 KV, 1.2×50 μS short-branch test.Varistor 372, resistors 374, 376 and 378, and capacitor 380 protect thepower supply from lightning strike surges.

A 550 VAC varistor 372 serves as the basis of the protection circuit. Ithas the lowest standard voltage that can handle a 528 VAC input. Thedevice has a maximum clamping voltage of 1500 volts at 50 amps.

A varistor placed directly across an AC line is subject to extremelyhigh surge currents and may not protect the circuit effectively. Highsurge currents can degrade the varistor and ultimately lead tocatastrophic failure of the device. Input resistors 374 and 376 limitthe surge currents to 35 amps. This insures that the clamping voltageremains below 1500 volts and extends the life of the varistor to tens ofthousands of strikes.

Resistor 378 and capacitor 380 act as an RC filter. The filter limitsthe rate of voltage rise at the output of the bridge rectifier. Thevoltage clamping circuit, transistors 344 and 346, is able to track thisreduced dv/dt. Current forced through diodes 382, 384 and capacitor 386(FIG. 5) is also controlled by the limited rate of voltage rise.

Resistors 374 and 376 are 1 watt carbon composition resistors. Theseresistors can withstand the surge energies and voltages. Resistor 378 isa flame-proof resistor that acts as a fuse in the event of a failure inthe remainder of the circuit.

The values of resistors 374, 376 and 378 are low enough so that they donot interfere with the operation of the power supply or dissipateexcessive amounts of power.

Finally it is noted that resistors 388 and 390 act to generate the powerfail voltage PF.

By using the wide voltage ranging of the invention, a single meter canbe used in both a four wire wye application as well as in a four wiredelta application. It will be recognized that a four wire deltaapplication includes 96V sources as well as a 208V source. In the pastsuch an application required a unique meter in order to accommodate the208V source. Now all sources can be metered using the same meter used ina four wire wye application.

While the invention has been described and illustrated with reference tospecific embodiments, those skilled in the art will recognize thatmodification and variations may be made without departing from theprinciples of the invention as described herein above and set forth inthe following claims.

What is claimed is:
 1. A meter for measuring electrical energy usage ofa circuit having a 3-wire delta service, comprising:a resistive dividermeans coupled to said circuit having a 3-wire delta service for reducinganalog voltages in each phase of said circuit having a 3-wire deltaservice and for scaling said analog voltages so reduced to provide anoutput of scaled voltage signals such that the peak-to-peak voltage ofsaid scaled voltage signals associated with each phase of said circuithaving a 3-wire delta service is less than about 1 volt; and aprecessing system coupled to said resistive divider means to receive aninput of said scaled voltage signals for processing said scaled voltagesignals to generate energy usage data representative of the amount ofenergy expended by said circuit having a 3-wire delta service.
 2. Themeter of claim 1, wherein said resistive divider means comprises:threeresistive divider circuits, each said resistive divider circuit beingcoupled between one phase of said circuit and a common point among saidthree resistive divider circuits, each said resistive divider circuitcomprising:a first circuit branch coupled to a respective one of saidphases of said circuit and providing most of the total resistance ofeach resistive divider circuit thereby limiting the current in saidfirst circuit branch to a predetermined acceptable watt loss and safetylevel; and a second circuit branch coupled to said first circuit branchfor scaling said analog voltage in said respective phase to generate arespective scaled voltage signal.
 3. The meter of claim 2, wherein eachsaid first circuit branch comprises two resistors in series.
 4. Themeter of claim 1, further comprising:a current sensing means coupled tosaid processing system for sensing the current in at least two phases ofsaid circuit and for generating current signals representative of thecurrent in each phase so sensed, wherein said processing system receivesan input of said current signals and uses said current signals inaddition to said scaled voltage signals to generate said energy usagedata.
 5. The meter of claim 2, wherein said first circuit branch has aresistance of at least about 1 MΩ, and said second circuit branch has aresistance of between about 100Ω and about 1 KΩ.
 6. The meter of claim1, wherein the resistive divider means comprises:a first resistivedivider having a resistance not less than approximately 2 MΩ and beingadapted to interface to a first phase of said circuit; a secondresistive divider having a resistance not less than approximately 2 MΩand being adapted to interface to a second phase of said circuit; and athird resistive divider having a resistance not less than approximately2 MΩ adapted to interface to a third phase of said circuit.
 7. The meterof claim 6, wherein each resistive divider comprises:a current limiterhaving at least two resisters of the same resistance and watt loss inseries; and a scaling resistor in series with the current limiter andhaving a resistance that does not exceed about 1 KΩ.
 8. The meter ofclaim 7, wherein said scaling resistor is a metal film resistor.
 9. Themeter of claim 1, wherein the scaled voltage signals are provided as adirect input from the resistive divider means to the processing systemsuch that the scaled voltage signals are not phase shift compensatedprior to processing by the processing system.
 10. An electronic meterfor measuring electrical energy usage of a circuit having a 3-wire deltaservice, comprising:a resistive voltage divider having a resistance ofat least about 1 MΩ and being coupled to the circuit having a 3-wiredelta service for limiting the current into the electronic meter and forscaling an analog voltage component in each phase of the electricalenergy to generate a scaled voltage signal having a predeterminedmaximum peak-to-peak value.
 11. The meter of claim 10, wherein saidresistive voltage divider comprises:three resistive divider circuits,each said resistive divider circuit being coupled between one phase ofsaid circuit and a common point among said three resistive dividercircuits, each said resistive divider circuit comprising:a first circuitbranch coupled to a respective one of said phases of said circuit forlimiting the current in said first circuit branch to a predeterminedacceptable watt loss and safety level; and a second circuit branchcoupled to said first circuit branch for scaling an analog voltagecomponent in said respective phase to generate a respective scaledvoltage signal.
 12. The meter of claim 11, wherein each said firstcircuit branch comprises two resistors in series.
 13. The meter of claim11, wherein the predetermined maximum peak-to-peak voltage is about 1 V.14. The meter of claim 11, wherein each said first circuit branch has aresistance of approximately 2 MΩ.
 15. The meter of claim 14, wherein thepredetermined maximum peak-to-peak voltage of about 1 V.
 16. The meterof claim 10, wherein said resistive voltage divider comprises 3resistive dividers, each being associated with one of the phases of theelectrical energy, each resistive divider comprising:a current limiterhaving at least two resisters of the same resistance and watt loss inseries; and a scaling resistor in series with the current limiter andhaving a resistance that does not exceed about 1 KΩ.
 17. The meter ofclaim 16, wherein the scaling resistor is a metal film resistor.
 18. Themeter of claim 16, wherein the scaled voltage signals have a linearvoltage with a minimal phase shift without phase shift compensation. 19.The meter of claim 18, wherein the scaled voltage signals have a linearvoltage with a minimal phase shift without phase shift compensation. 20.The meter of claim 2, wherein said first circuit branch has a resistanceof at least about 2 MΩ, and said second circuit has a resistance ofbetween about 100Ω and about 1 KΩ.
 21. An electronic meter for measuringelectrical energy usage of a circuit having a 3-wire delta service,comprising:a resistive voltage divider having a resistance of at leastabout 2 MΩ and being coupled to the circuit having a 3-wire deltaservice for limiting the current into the electronic meter and forscaling an analog voltage component in each phase of the electricalenergy to generate a scaled voltage having a predetermined maximumpeak-to-peak value.